Vacuum cleaner appliances capable of picking up both wet and dry material, commonly referred to as wet/dry vacuums or wet/dry vacs, are well-known. Wet/dry vacs are often used in workshops and other environments where both wet and dry debris can accumulate.
Wet/dry vacs conventionally consist of a collection tank or canister, often mounted on wheels or casters, and a cover or lid upon which a motor and impeller assembly is mounted. The motor and impeller assembly creates a suction within the canister, such that debris and liquid are drawn in to the canister through an air inlet to which a flexible hose can be attached. A filter within the canister prevents incoming debris from escaping from the canister while allowing filtered air to be forcibly expelled through an air outlet. One example of a such a wet/dry vac is shown in U.S. Pat. No. 4,797,072.
Prior art examples of wet/dry vacuums include: U.S. Pat. No. 5,548,868 to Berfield et al., entitled "Pilot and Detent Apparatus for a Vacuum Device;" U.S. Pat. No. 5,535,500 to Stephens et al., entitled "Method for Manufacturing a Bucket for a Wet/Dry Vacuum;" U.S. Pat. No. 5,598,605 to Tomasiak, entitled "Wet/Dry Utility Vacuum with a Wheel Mount;" U.S. Pat. No. 5,555,600 to Corson, entitled "Non-Tipping Wet/Dry Vacuum;" U.S. Pat. No. 5,606,769 to Tomasiak, entitled "Wet/Dry Utility Vacuum Cleaner with Detachable Blower;" U.S. Pat. No. 5,608,945 to Crouser, et al., entitled "Wet/Dry Utility Vacuum Cleaner;" and U.S. Pat. No. 5,611,107 to Tomasiak et al., entitled "Latching Mechanism for Wet/Dry Utility Vacuum Cleaner with Detachable Blower."
A typical wet/dry vac motor and blower assembly comprises a motor having a closed-face, multiple-blade blower wheel or impeller disposed on a drive shaft thereof. The motor and blower assembly is typically disposed in a collection canister lid assembly, with the rotating blower wheel disposed within a blower chamber, sometimes referred to as a collector chamber. The collector chamber is accessed via an air intake, such that a suction created by rotation of the impeller within the collector chamber causes air to be drawn into the air intake.
In some designs, the motor and blower assembly is adapted to be capable of detachment from the lid of the collection canister, thereby being usable as a hand-held blower for blowing dust and debris, such as in a workshop, outdoor area, or the like.
A conventional wet/dry vac, whether of the detachable or fixed power head type, has two air flow systems. A first air flow system is established for cooling the electric motor. The second air flow system is the blower wheel or impeller airflow, which affects the suction performance of the vac (and the blowing performance, for those vacs which are adaptable or convertible between and vacuum and a blower). It is the latter airflow system to which the present invention is primarily applicable, and unless otherwise noted herein, the terms "airflow" and "airflow system" shall be intended to refer to the blower wheel or impeller airflow of a vac.
Typically, the motor for a wet/dry vac operates at relatively high speeds, on the order of 20,000 revolutions per minute (RPM). Those of ordinary skill in the art will appreciate that such motors can be very noisy in operation. The noise is heightened by the effects of the bladed impeller turning at the speed of the motor, and by the airflow through the vac. The most predominant noise made by vacs having a scroll-type collector chamber occurs at or around a specific audio frequency, referred to as the "blade passing frequency." The blade passing frequency is computed according to the following formula: EQU Blade Passing Frequency=Rotation of Impeller(revolutions per second).times.Number of Impeller Blades.
Thus, for a vac having a motor which turns at 19,000 to 20,000 revolutions per minute (i.e., approximately 316.66 to 333.33 revolutions per second) and having an impeller or blower wheel with seven blades, the blade passing frequency is on the order of approximately 2217 to 2333 Hz. This is well within the range of human audibility, and the noise level at this pitch can be irritating indeed.
It has heretofore been generally understood that the more obvious steps that can be taken to reduce or minimize the noise level of an operating vac have a deleterious effects upon the operation and performance of the vac. For example, reducing the speed of the motor, reducing the number of impeller blades, or reconfiguring the geometry of the airflow path to reduce the velocity of the air would all tend to degrade the performance of the vac, in terms of the suction strength and volume of air moved.